I can't imagine them being hauled off somewhere. But I guess if you don't own property, what would you do.
Not every property has adequate soil cover to effectively bury a horse, and in some locations it is illegal. We also have a small stream running through the property. Animals should not be buried where the decomposing drugs used to euthanize will enter the surface water. I will try to locate the source of information about composting horses.
There is also a debate about the methods used to euthanize. Barbituates are more aethetically pleasing for the owner, but place alot of chemical compounds in the ground. The Europeans typically use gunshot or captive bolt to euthanize. It is just as quick as barbituates, but doesn't pollute when the animal is buried.
Just a note about the HSUS list. It is already out of date for my area. Horse owners should make a phone call to their county extension, and then to the supplier of services. Know your options, and the cost.